The use of optical time domain reflectometers to detect and locate a fault in a fiber optic cable is well known. For example, U.S. Pat. No. 4,749,247, issued June 7, 1988 to Scott F. Large, discloses a self-monitoring link which utilizes a GRIN lens/blazed grating combination to multiplex an OTDR signal with data signals on a fiber optic communication link. A reference OTDR signature and a continuously determined average OTDR signature are compared and an alarm signal is generated when the continuously determined signature differs by a preselectable amount from the reference signature, a microprocessor being provided to adjust the preselectable limit in response to the operational parameters of the communication link.
U.S. Pat. No. 4,875,772, issued Oct. 24, 1989 to John R. Gentile, discloses a system which enables a single optical time domain reflectometer to monitor a plurality of cables. In one embodiment, this is achieved by using two lasers having different wavelengths, which are separately fired into different cables, the returning signals being fed to the appropriate port of a wavelength dependent coupler, whereas in another embodiment each laser simultaneously checks two cables having different lengths, the length difference enabling identification of the cable in which the fault has occurred.
However, an optical time domain reflectometer can detect only backscattered light. Such light represents only about one percent of the total light power transmitted through a fiber and, therefore, is of very low power and consequently difficult to detect. This difficulty can lead to confusion of the results or, if steps are taken to avoid such confusion, to reduced sensitivity. The electronic detection devices used, which are usually avalanche photodiodes, have a noise threshold such that, as a signal to be detected is reduced in amplitude, the probability of the detector registering the signal as noise instead of the signal itself increases. This leads to several disadvantages.
In commercially available optical time domain reflectometers, this problem is mitigated by digital sampling techniques which gather a predetermined number of samples of the signal. The results are then processed to extract the signal from the noise. The closer the signal level approaches the noise floor of the device, the more difficult this process becomes. For example, some commercially available optical time domain reflectometers sample over a twenty minute period, which is clearly unacceptable in many circumstances and, in particular, in security situations where it is important to detect rapidly any unauthorized attempt to collect data from a cable.
Thus, if an event occurs which causes a small change in the attenuation of the fiber or a splice, detection using an optical time domain reflectometer will always be considerably more difficult, since the change can easily be confused with noise inherent in the detection device. This leads to uncertainty of operation and requires interpretation of the results, based on probability.
Another difficulty is that the signal is attenuated by normal attenuation of the fiber being tested or monitored. Therefore, the further the light pulse has to travel, the lower the light level will be and, therefore, the closer the signal approaches the noise floor of the detector. Consequently, interpreting a feature or change in features at some distance away from the monitoring point becomes even more difficult.
Also, it has been found that variations in the diameter of an optic fiber can be misinterpreted as attenuation effects when using an optical time domain reflectometer. Such diameter variations may have a stable transmission loss but might exhibit an unstable backscatter signature under operating conditions.
For these various reasons, it is preferable to avoid the use of an optical time domain reflectometer for detecting anomalies, as distinct from determining the location of such anomalies.
U.S. Pat. No. 4,893,006, issued Jan. 9, 1990 to Toshiyuki Wakai et al, and U.S. Pat. No. 4,898,463, issued Feb. 6, 1990 to Takashi Sakamoto et al, disclose the use of optical time domain reflectometers for locating anomalies, with means for simplifying the manual operation involved in controlling the apparatus.
There is also commercially available a fiber optic alarm apparatus, marketed by Plexcom, Inc., of Simi Valley, Calif. under the trade name 911 FIBEROPTIC ALARM, which continuously monitors the integrity of a cable by sending light down one fiber to a remote end of a cable, recoupling the light there to another fiber for retransmission through the cable and checking the amount of received light.
This prior apparatus compensates for slow changes in light level, caused for example by temperature changes, component aging, loosening of connectors, etc., but remains sensitive to rapid changes in light level.
It is a disadvantage of this prior apparatus that it does not enable the location of a detected anomaly to be effected. Such location of the anomaly therefore requires wastage of true and consequential downtime and loss of revenue for data transmission, while separate apparatus is employed to locate the anomaly.